For vision, insect and vertebrate eyes use rhabdomeric and ciliary photoreceptor cells, respectively. These cells show distinct architecture and transduce the light signal by different phototransductory cascades. In the marine rag-worm Platynereis, we find both cell types: rhabdomeric photoreceptor cells in the eyes and ciliary photoreceptor cells in the brain. The latter use a photopigment closely related to vertebrate rod and cone opsins. Comparative analysis indicates that both types of photoreceptors, with distinct opsins, coexisted in Urbilateria, the last common ancestor of insects and vertebrates, and sheds new light on vertebrate eye evolution.

Platynereis dumerilii, a marine polychaetous annelid with indirect development, can be continuously bred in the laboratory. Here, we describe its spectacular reproduction and development and address a number of open research problems. Oogenesis is easily studied because the oocytes grow while floating in the coelom. Unlike the embryos of other model spiralians, the Platynereis embryo is transparent giving insight into the dynamic structures and processes inside the cells that accompany the prevailing anisotropic cleavages. Functional studies on cell specification and differential gene expression in embryos, larvae, and later stages are underway. Lifelong proliferation of uniform trunk segments qualifies Platynereis as a model for the study of gene expression and of the functional circuitry of this process. Platynereis can also become a stepping stone in the comparison of segmentation between annelids and arthropods because it comes closer to the putative ancestral morphology and style of development than other model annelids.

Following an enzymatic procedure for softening the egg envelope, blastomeres in the embryo of the polychaete Platynereis dumerilii were injected with TRITC-dextran. Injection was successful in the following blastomeres: AB, CD, A, B, C, D, 1a-1d, 1A-1D, 4d, and 4d(1). The distribution of fluorescent label was recorded by confocal laser scanning microscopy of young, three-segmented worms after 3 or 4 days of development, in some cases also in 1-day-old trochophore larvae. Results were documented by single optical sections, by stacking a limited number or a complete set of optical sections, and by computer-generated surface views of both the labeled tissue domains and the body contours from complete image stacks of whole worms. With respect to their descent from the embryonic cell pattern, five major compartments can be distinguished which together compose the body of the young worm: 1) The epispheric, epidermal, and neural region of the head, composed of four domains arranged as quasi-radial sectors derived from micromeres 1a, 1b (left and right ventral), and 1c and 1d (right and left dorsal). 2) A posttrochal epidermal region of the head originating from micromeres 2a(1)-2c(1) and constituting the ventral and lateral posttrochal epidermis of the head. 3) A stomodeal-ectomesodermal region of the head, including the stomodeum (micromeres 2a(2) and 2c(2)), its mesodermal envelope, and head mesoderm (micromeres 3a-3d). 4) A solid cone composed of the four terminal macromeres 4A-4D, forming the core of the trunk as the endoderm anlage. 5) An epidermal and mesodermal coating of the trunk originating from the dorsal micromeres 2d and 4d. The region of the so-called (first, anterior) peristomial cirri at the posterior flanks of the head is also composed of 2d- and 4d-derived trunk tissue, thus corroborating the postulated descent of this region and its appendages from a cephalized anteriormost trunk segment and its parapodia. The cell-lineage domains of the first and third micromere tiers are arranged left or right of the sagittal plane, while two micromeres of the second quartet are in a lateral and, initially, two in a median position (2b ventral and 2d dorsal). The offspring of micromere 2d expand from a dorsal position toward the ventral midline and those of cell 4d from a posterior-dorsal site toward the anterior, initially forming two lateral bands. In the epispheric part of the head, part of the neurectodermal tissue derived from micromeres 1a and 1b interweaves in a medio-sagittal bar, and part of the first micromere offspring of all four quadrants (1a-1d) combine in forming a central brain neuropil. Each of the latter sends neurites through both of the circumesophageal connectives. Paired muscle tracts extend through the head toward the base of the antennae and are probably derived from micromeres 3a and 3b. A mesodermal envelope of the stomodeum is probably built by the 3c and 3d micromeres. The formation of symmetry and the nature of the body axes in the embryo and adult of Platynereis dumerilii are discussed....